Genetic studies have indicated that genes controlling the isoelectric focusing (IF-) spectra of normal mouse immunoglobulin light chains are closely linked to the Ly-2,3 locus on Chromosome 6. In the present work we will concentrate on the differences in light chain spectra expressed by normal BALB/c and NZB mice. These differences appear to involve discrete "bands" in the normal light chain isoelectric focusing profile. We have identified a total of 8 BALB/c myelomas having light chains co-focusing with the two major polymorphic bands of normal light chains. Out of a total of 125 NZB myelomas investigated, we found none with light chains co-focusing precisely with the polymorphic bands. One hypothesis is that NZB mice lack the structural (v-) genes coding for that specific subgroup of light chains. We are currently sequencing 3 of the BALB/c light chains. Analysis of the pattern of sequence variation within the group of proteins will provide a picture of the nature of variability within a group of kappa chains probably coded by a single V-gene. The partial sequences so far obtained indicate that the 3 light chains do indeed form an extremely homologous group, analogous to the VK21A subgroup. In addition to the structural work we are continuing to look for evidence of recombination between the two light chain loci Igk-Ef1 and Igk-Ef2. A third objective will be to produce a subgroup-specific antiserum for use in a RIA for the light polymorphism. This could be used to (a) aid genetic studies (b) to quantitate the expression of the Ef2 marker light chains in various inbred strains.